Fluid-pressure-operated motors

ABSTRACT

A fluid-pressure-operated reciprocating motor having a spool and piston slidable in spool and piston cylinders respectively in which the spool has a first pair of opposed faces which respectively communicate with a pair of spool-working chambers, and a second pair of opposed faces which respectively communicate with a pair of spool-biasing chambers, and the piston is double acting and has a pair of opposed faces which respectively communicate with a pair of piston chambers, reversal of the piston being initiated by reversal of the spool which serves to connect one piston chamber alternately to fluid pressure and to exhaust and to connect the other piston chamber alternately to exhaust and to fluid pressure respectively; in which the timing of reversal of the spool is determined according to the position of the piston in its cylinder, and the motor is so arranged that, with the spool at an end of its stroke, one spool-working chamber is open to pressure while the other spool-working chamber is open to exhaust to urge the spool to that end and one spool-biasing chamber is open to exhaust and, to reverse the spool, fluid pressure in both spool-working chambers is caused to apply equal and opposite axial thrusts on the spool and pressure is caused to be admitted to the other spool-biasing chamber in opposition to the load on the spool applied by fluid pressure in said one spool-working chamber.

United States Patent Butterworth 1 Feb. 22, 1972 [54]FLUID-PRESSURE-OPERATED MOTORS FOREIGN PATENTS OR APPLICATIONS 1,923,741l/l970 Germany ..9l/306 Primary Examiner-Paul E. MaslouskyAttamey-Stevens, Davis, Miller & Mosher [57] ABSTRACT Afluid-pressure-operated reciprocating motor having a spool and pistonslidable in spool and piston cylinders respectively in which the spoolhas a first pair of opposed faces which respectively communicate with apair of spool-working chambers, and a second pair of opposed faces whichrespectively communicate with a pair of spool-biasing chambers, and thepiston is double acting and has a pair of opposed faces whichrespectively communicate with a pair of piston chambers, reversal of thepiston being initiated by reversal of the spool which serves to connectone piston chamber alternately to fluid pressure and to exhaust and toconnect the other piston chamber alternately to exhaust and to fluidpressure respectively; in which the timing of reversal of the spool isdetermined according to the position of the piston in its cylinder, andthe motor is so arranged that, with the spool at an end of its stroke,one spool-working chamber is open to pressure while the otherspool-working chamber is open to exhaust to urge the spool to that endand one spool-biasing chamber is open to exhaust and, to reverse thespool, fluid pressure in both spool-working chambers is caused to applyequal and opv posite axial thrusts on the spool and pressure is causedto be admitted to the other spool-biasing chamber in opposition to theload on the spool applied by fluid pressure in said one spool-workingchamber.

27 Claims, 2 Drawing Figures FLUlD-PRESSURE-OPERATED MOTORS Thisinvention relates to fluid-pressure-operated motors and is particularly.directed to such a motor for producing reciprocatory motion.

It has hitherto been proposed to provide a fluid-pressureoperated motorof the type in which a spool, axially slidable in a spool cylinder,controls reciprocation of a piston which is axially slidable in a pistoncylinder and the position of the piston in its cylinder controls axialmovement of the spool. In such motors the piston is generally doubleacting and movement of the spool affected by variations in fluidpressure applied to a pair of opposed faces on the spool.

In accordance with the present invention there is provided afluid-pressureoperated motor for producing reciprocating motion whichincludes;

a spool axially slidable in a spool cylinder, the spool having a firstpair of opposed faces each of which respectively communicates with oneof a pair of spool-working chambers in the spool cylinder, and a secondpair of opposed faces each of which respectively communicates with oneof a pair of spoolbiasing chambers in the spool cylinder;

a piston axially slidable in a piston cylinder, the piston being doubleacting and having a pair of opposed faces each of which respectivelycommunicates with one of a pair of piston chambers, reversal of thepiston being initiated by reversal of the spool which serves to connectone piston chamber alternately to fluid pressure and to exhaust and toconnect the other piston chamber alternately to exhaust and to fluidpressure respectively;

and wherein the timing of reversal of the spool is determined accordingto the position of the piston in its cylinder, the arrangement beingsuch that with the spool at an end of its stroke, one spool-workingchamber is open to fluid pressure while the other spool-working chamberis open to exhaust to urge the spool to that end and one spool-biasingchamber is open to exhaust and, to reverse the spool, fluid pressure inboth spool-working chambers is caused to apply equal and opposite axialthrusts on the spool and fluid pressure is caused to be admitted to theother spool-biasing chamber in opposition to the load on the spoolapplied by fluid pressure in said one spool-working chamber.

It is to be realized that by the term fluid as used throughout thisspecification we include both liquids (hydraulically operated motors)and gases (pneumatically operated motors).

To reverse the spool, fluid pressure in both spool-working chambers isso arranged that the axial thrusts on the spool from such fluid pressureor pressures in the spool-working chambers are equal and opposite, andmovement of the spool from its end of stroke position is achieved byapplying fluid pressure in said other spool-biasing chamber while saidone spool-biasing chamber is open to exhaust. Equal and opposite axialthrust on the spool from the spool-working chambers may be achieved byhaving the first pair of opposed faces of the spool of differenteffective area and by providing a corresponding difference in fluidpressure which is applied to each of the pair of spool-working chambers.For example, if the effective areas of the first pair of opposed facesof the spool are in the ratio 2: l then the fluid pressure in thespoolworking chambers are so arranged that, at reversal of the spool,fluid pressure in the spool-working chamber which communicates with theface of smaller effective area is in the ratio of 2:1 with fluidpressure in the spool-working chamber which communicates with the faceof larger effective area, thereby providing equal and opposite axialthrust on the piston. However, in a preferred embodiment the first pairof opposed faces of the spool are of equal effective area, the secondpair of opposed faces of the spool each have an effective area less thanthe effective area of one of the first pair of opposed faces, and thepair of opposed faces of the piston are of equal effective area; thearrangement being such that, to reverse the spool, fluid pressure iscaused to be admitted to the said other spool-biasing chamber inopposition to the load on the spool applied by fluid pressure in saidone spool-working chamber and fluid pressure in both spool-workingchambers is caused to be equalized.

The sequence of fluid pressurization and exhausting of the spoolchambers to achieve reversal of the spool may conveniently follow eitherof two courses. In a first and preferred arrangement, with the spool atan end of its stroke, one spoolworking chamber is open to fluid pressurewhile the other spool-working chamber is open to exhaust to urge thespool to that end and one spool-biasing chamber is open to exhaust,fluid pressure is first caused to be admitted during axial movement ofthe piston to the other spool-biasing chamber in opposition to the loadon the spool applied by fluid pressure in said one spool-working chamberand thereafter, at or towards the end of stroke of the piston consistentwith its axial movement, fluid pressure in both spool-working chambersis caused to apply equal and opposite axial thrusts on the spool tocause the spool to reverse. By a further arrangement, with the spool atan end of its stroke, one spool-working chamber is-open to fluidpressure while the other spool-working chamber is open to exhaust tourge the spool to that end and one spool-biasing chamber is open toexhaust, during axial movement of the piston, fluid pressure in bothspool-working chambers is first caused to apply equal and opposite axialthrusts on the spool and thereafter, at or towards the end of stroke ofthe piston consistent with its axial movement, fluid pressure is causedto be admitted to the other spool-biasing chamber in opposition to theload on the spool applied by fluid pressure in said one spool-workingchamber to reverse the spool.

Preferably each of the pair of piston chambers is respectively inconstant communication with one of the pair of spoolworking chambers andeach piston chamber has associated therewith a port through which it isadapted to be exhausted. During axial movement of the piston, the portthrough which the contracting piston chamber is exhausting is adapted tobe closed by the piston towards its end of stroke and thereafter fluidpressure in the contracting piston chamber and spoolworking chamber incommunication therewith applies an axial thrust to the spool whichequalizes the opposing axial thrust applied to the spool by fluidpressure in the expanding piston chamber and spool-working chamber incommunication therewith. By having the first pair of opposed faces ofthe spool of equal effective area, the second pair of opposed faces ofthe spool of equal effective area, and the pair of opposed faces of thepiston of equal effective area, when the port through which thecontracting piston chamber is exhausting is closed by axial movement ofthe piston, it will be apparent that fluid pressure in bothspool-working chambers and piston chambers is at least equalized byequal and opposite reaction through the piston. However, it should beborne in mind that the inertia of the moving piston may impart apressure impulse to fluid in the contracting piston chamber when theport through which it is exhausting is closed thereby imparting amomentarily higher fluid pressure in the contracting piston chamber andspool-working chamber in communication therewith than the fluid pressurein the other piston chamber and spool-working chamber. As such, whenusing relatively heavy piston, it is preferable that the equalization offluid pressure in the spool-working chambers (or thrust on the spool inboth axial directions) causes the spool to reverse so that the pressureimpulse developed by loss of inertia of the piston at the end of itsstroke imparts a kick" to the spool to lift it from its end of strokeposition.

Although fluid pressure in both spool-working chambers to apply equaland opposite axial thrusts on the spool may be achieved, as abovedescribed, by imparting an equal and opposite reaction through thepiston to fluid in the piston chambers, when the port through which thecontracting piston chamber exhausts is closed, it will be apparent thatif the piston is located at an extreme end of its stroke (for examplewhen abutting the end of its cylinder) such equal and opposite reactionthrough the piston is not possible and the motor may stall. To avoidsuch a possibility, in addition or as an alternative to the provision ofequal and opposite reaction through the piston, equal and opposite axialthrusts on the spool may be achieved by use of auxiliary valve meanswhich is adapted to be actuated when the piston is located at or towardsan end of stroke position. With the spool at an end of stroke position,actuation of the auxiliary valve means admits fluid pressure to thespool-working chamber which when pressurized, urges the spool to itsother end of stroke position. Preferably such auxiliary valve means isadapted to be actuated during axial movement of the piston to admitfluid pressure to the respective spool-working chamber only when thepiston is located at a position in which the .port through which thecontracting piston chamber exhausts is closed by the piston and prior tothe piston abutting the end of its cylinder. The auxiliary valve meansconveniently comprises ports in the piston cylinder which are adapted tocommunicate with fluid pressure and are controlled by cooperating landson the piston (or piston rod) so that when the piston is at one or otherextreme end of its stroke a port in communication with fluid pressure isopened to communicate with the respective spool-working chamber.

In a preferred embodiment, with the motor in a condition in which thespool is located at an end of its stroke, one spoolworking chamber opento fluid pressure, the other spoolworking chamber open to exhaust andone spool-biasing chamber open to exhaust, fluid pressure is caused tobe admitted to the other spool-biasing chamber in opposition to the loadon the spool applied by fluid pressure in said one spool working chamberby way of valve means responsive to axial movementof the piston. In theaforementioned construction of the motor in which a port through whichthe contracting piston chamber exhausts is closed by the piston duringits axial movement, the valve means is preferably adapted to be actuatedto admit fluid pressure to the said other spool biasing chamberimmediately before theport through which the contracting piston chamberexhausts is closed by the piston; consequently, when the valve means isactuated, the spool will subsequently reverse on fluid pressure in bothspool-working chambers being caused to apply equal and opposite axialthrusts on the spool. Alternatively the valve means may be arranged tobe actuated to admit fluid pressure to the said other spool-biasingchamber to cause the spool to reverse after fluid pressure in bothspool-working chambers has applied equal and opposite axial thrust onthe spool. Conveniently the valve means comprises cooperating ports andland between the piston and its cylinder or between a piston rod andbore through which it is slidable.

In a further embodiment, with the motor in a condition in which thespool is located at an end of its stroke, one spoolworking chamber opento fluid pressure, the other spoolworking chamber open to exhaust andone spool-biasing chamber open to exhaust, fluid pressure is caused tobe admitted to the other spool-biasing chamber in opposition to the loadapplied by fluid pressure in said one spool-working chamber by way ofbleed passage means which is in constant communication with said otherspool-biasing chamber and is adapted to communicate with fluid pressureduring expansion of a piston chamber consistent with the spool at saidend and to communicate with exhaust during contraction of a pistonchamber consistent with the spool at the other end of its stroke.

Two control ports may be provided in the spool cylinder each of which isadapted to respectively communicate with one of the pair ofspool-biasing chambers. The control ports are controlled byreciprocation of the spool to be alternately opened and closed to theirrespective spool-biasing chambers and, with the spool at an end of itsstroke, one control port is open and the other control port is closed toits respective spool-biasing chamber, the closed control port beingassociated with the spool-biasing chamber which, if subjected to fluidpressure, would urge the spool away from its said end of strokeposition. The control ports are adapted to communicate alternately withfluid pressure and exhaust during reciprocation of the piston so that,with the spool at an end of its stroke, when the piston reaches an endof its stroke consistent with the spool in said end of stroke position,the closed control port communicates with fluid pressure and the opencontrol port (and thereby its respective spool-biasing chamber)communicates with exhaust. Upon reversal of the spool, initial movementof the spool from its end of stroke position causes the control port incommunication with fluid pressure to be opened to its respectivespool-biasing chamber to urge the spool to its other end of strokeposition. Conveniently the aforementioned valve means is adapted tocontrol communication between each control port and its respectivelyassociated spool-biasing chamber so that, when a control port is open tofluid pressure, actuation of said valve means causes communication to bemade between said control port and its respective spool-biasing chamber.Alternatively the aforementioned bleed passage means may comprise smallbleed passages which are in constant communication between each controlport and its respectively associated spool biasing chamber.

It may be possible for the motor to be assembled or come to rest in aposition in which both the spool-working chambers, biasing chambers andthe piston chambers communicate with fluid pressure. If the first pairof spool faces are of equal effective area, the second pair of spoolfaces are of equal effective area and the pair of opposed piston facesare of equal effective area it will be apparent that in such conditionhydraulic equilibrium may result on the spool which could cause themotor to stall. To avoid such a possibility the second pair of opposedspool faces are preferably made with different effective areas andconsequently even if each of the spool working and biasing chamberscommunicate with fluid pressure the spool will be biased to one or otherend position of its stroke.

Two embodiments of the present invention will now be described, by wayof example only, with reference to the accompanying illustrativedrawings in which:

FIG. 1 shows a fluid pressure operated motor constructed in accordancewith the present invention and in part section taken along thelongitudinal axis of the spool and piston, and

FIG. 2 illustrates a further, and preferred embodiment of the invention.

Referring firstly to FIG. 1, the motor includes a body part 1 whichdefines a spool cylinder shown generally at 2 and a body part la whichdefines a piston cylinder 3. The spool cylinder 2 is stepped to provideend parts 4 and 4a of equal diameter and adjacent midparts 5 and 6.Spool cylinder part 4 communicates with part 5 and cylinder part 5 is oflarger diameter than cylinder part 4. Spool cylinder part 411communicates with cylinder part 6 and cylinder part 6 is of largerdiameter than cylinder part 5. Axially slidable in the spool cylinder 2is a spool shown generally at 7 which comprises end lands 8 and 9 whichare respectivelyslidable in cylinder end parts 4 and 4a and lands l0 andl l which are respectively slidable in cylinder parts 5 and 6. One endof the spool 7 is provided with a rod 12 and the end of the spoolcylinder 2 remote from the rod 12 is provided with an abutment 13. Theend face of the spool 7 adjacent the rod 12 defines with its adjacentend of the spool cylinder part 4 a spool-working chamber 12a and theopposite end face of the spool 7 (adjacent the abutment l3) defines withits adjacent end of the spool cylinder part 40 a second spool-workingchamber 13a. The rod 12 and abutment 13 provide physical limits to axialmovement of the spool in its cylinder and may be castellated to ensurethat fluid under pressure can flow over the ends of the spool when it islocated at one or other end of its stroke in the spool cylinder. Infact, in the embodiment described, the physical limitation of stroke isonly precautionary, since the hydraulic operation is so arranged thatthe stroke is limited hydraulically.

The spool land 10 has a side face 14 which is located adjacent the spoolland 8 and partly defines with the adjacent stepped end of the cylinderpart 5 a spool-biasing chamber 15. Similarly, the spool land 11 has aside face 16 which is located adjacent the spool land 9 and partlydefines with the adjacent stepped end of the cylinder part 6 a furtherspool-biasing chamber 17. The spool lands and 11 have opposed side faces18 and 19 respectively which define, with the adjacent ends of cylinderparts 5 and 6, a spool transfer chamber 20.

Slidably housed in the piston cylinder 3 is a piston shown generally at21 having rods 21a extending axially and in sealed manner through bores21b in the end walls of the body part In and a pair of axially spacedlands 22 and 23. Defined between the opposed side faces of the lands 22and 23 and part length of the piston cylinder 3 is a piston transferchamber 24; defined between the piston land 22 and one end part of thepiston cylinder 3 is a first piston chamber 25; and defined between thepiston land 23 and the other end part of the piston cylinder 3 is asecond piston chamber 26. An annular recess 27 is provided in the pistonrod 21a adjacent the piston land 22 and on the side thereof in thechamber 25; similarly a further annular recess 28 is provided in thepiston rod 21a adjacent the piston land 23 and on the side thereof inthe chamber 26. The annular recesses 27 and 28 are so arranged that inthe unlikely event of the piston 21 being located at one or otherextreme end of its stroke (which is ultimately determined by the pistonlands 22 and 23 abutting their respective ends of the piston cylinder 3)the annular recess 27 will be in communication with a port 30 in thebore 21b or the annular recess 28 will be in communication with a port31 in the bore 21b. Normally the ports 30 and 31 are closed by the fulldiameter of the piston rods 21a.

The motor is provided with an inlet port 32 which is adapted to beconnected to a source of fluid (preferably hydraulic) under pressure.The inlet port 32 communicates through passage 33 with the port 30 inthe bore 21b; through passage 34 with the port 31 in the bore 21b;through a passage 35 with a pressure inlet port 36 which is located inthe spool cylinder part 4; and also by way of a branch passage 37 with apressure inlet port 38 which is located in the spool cylinder part 4a.

The ports 36 and 38 are so located that, at one end of stroke of thespool 7, the port 36 is closed by the spool land 8 whilst the port 38communicates with the spool-working chamber 13a; at the other end ofstroke of the spool, the port 36 communicateswith the spool workingchamber 12a while the port 38 is closed by the spool land 9, and in noposition of the spool are both ports 36 and 38 simultaneously closed byspool lands 8 and 9 to the spool-working chambers 12a and 13arespectively.

Located in the spool cylinder part 4 to be in permanent communicationwith the spool chamber 12a is a port 41 which communicates through apassage 42 with a port 43 opening into the piston chamber 25 at one endof the piston cylinder 3. Located in the spool cylinder part 4a to be inpermanent communication with the spool chamber 13a is a port 44 whichcommunicates through a passage 45 with a port 46 opening into the pistonchamber 26 at the end of the piston cylinder 3 remote from the port 43.

The motor is provided with an exhaust port 47 which communicates througha port 48 with the spool cylinder 2. The port 48 is located to be inconstant communication with the spool transfer chamber 20. Also locatedin the spool cylinder 2 to be in permanent communication with thetransfer chamber 20 is a port 49 which communicates through a passage 50with a port 51 in the piston cylinder 3. The port 51 is positionedintermediate the ports 43 and 46 and is located to be in permanentcommunication with the piston transfer chamber 24.

Located in and towards the end of the spool cylinder part 5 adjacent thespool cylinder part 4 is a control port 52 which communicates through apassage 53 with a port 54 located in the piston cylinder 3 between theports 51 and 46. Located in and towards the end of the spool cylinderpart 6 adjacent the spool cylinder part 4a is a further control port 55which communicates through a passage 56 with a port 57 located in thepiston cylinder 3 between the ports 51 and 43.

The control port 52 is controlled by the spool land 10 and the controlport 55 is controlled by the spool land 11; the spool lands 10, 11 andcontrol ports 52, 55, are so arranged that, at one end of stroke of thespool, the control port 52 is closed by the spool land 10 and thecontrol port 55 commu' nicates with the spool-biasing chamber 17; at theother end of stroke of the spool, the control port 55 is closed by thespool land 11 and the control port 52 communicates with the spoolbiasingchamber 15; and in no position of the spool are both control ports 52and 55 simultaneously closed by the lands l0 and 11 to the spool-biasingchambers 15 and 17 respectively.

The spool cylinder has a port 58 which is situated in the spool cylinderpart 5 and is located between the ports 49 and 52; the port 58communicates through a passage 59 with a port 60 in the piston cylinder3, the port 60 being located between the ports 57 and 43 and towards theport 43. Provided in the spool cylinder part 6 is a port 61 which islocated between the ports 49 and 55 and communicates through a passage62 with a port 63 in the piston cylinder 3, the port 63 being locatedbetween the ports 54 and 46 and towards the port 46. Ports 58 and 61 arecontrolled to be opened and closed to communication with the transferchamber 20 by spool lands l0 and 11 respectively. The spool lands 10, 11and ports 58, 61 are so arranged that, at one end of stroke of thespool, the port 58 communicates with the transfer chamber 20 while theport 61 is closed by the spool land 11; at the other end of stroke ofthe spool, the port 61 communicates with the transfer chamber 20 whilethe port 58 is closed by the spool land 10; and in no position of thespool do both ports 58 and 61 simultaneously communicate with thetransfer chamber 20. The spool lands 9, l1 and ports 38, 61 are soarranged that when port 38 is open to chamber 13a port 61 is closed byland 11, when port 61 is open to chamber20 port 38 is closed by land 9,and in no position of the spool do both ports 38 and 61 simultaneouslycommunicate with chambers 13a and 20 respectively. Similarly the spoollands 8, 10 and ports 36, 58 are so arranged that when port 36 is opento chamber port 58 is closed by land 10, when port 58 is open to chamber20 port 36 is closed by land 8, and in no position of the spool do bothports 36 and 58 simultaneously communicate with chambers 12a and 20respectively.

Provided in the motor are two small bleed passages 64 and 66 of whichbleed passage 64 is in constant communication, at one end with thespool-biasing chamber 15, and at its other end with the passage 53; andbleed passage 65 is in constant communication, at one end with thespool-biasing chamber 17, and at its other end with the passage 56, Thebleed passages 64 and 65 provide means whereby, when fluid underpressure is present in one or other of the passages 53 and 56, fluid canalways seep into one or other of the spool-biasing chambers 15 and 17.Although, in the present embodiment the bleed passages 64 and 65 areshown as being formed by conduits in the body part 1, they can beprovided by suitable machining or porting of the spool lands 10 and 11respectively or by machining the spool cylinder parts 5 and 6 topredetermined tolerances to provide the required seepage from the ports52 and 55 to the spool-biasing chambers 15 and 17 respectively.

The various diameters of the spool 7 are so arranged that the effectivearea at the end of the spool in the spool chamber 12a is equal to theeffective area at the end of the spool in the spool chamber 13a; theeffective pressure area of the spool side face 14 in the spool-biasingchamber 15 (which, if the chamber 15 contained fluid under pressurewould cause the spool to be biased rightwardly in the drawing) isslightly less than the effective area of the spool side face 16 in thespoolbiasing chamber 17 (which, if the chamber 17 contained fluid underpressure would bias the spool leftwardly in the drawing); and theeffective area of the side face 16 in the spoolbiasing chamber 17 isslightly less than the effective area of the end of the spool in eitherchambers 12a or 13a.

The ports 43 and 46 in the piston cylinder 3 are so arranged that whenthe piston 21 is located at one or other end of its cylinder (i.e., whenthe side face of land 22 abuts the end of the piston cylinder 3 or theside face of land 23 abuts the other end of the piston cylinder), theport 43 or 47 which is adjacent the end of the piston cylinder againstwhich the piston abuts, communicates by way of annular recess 27 or 28with its adjacent port 30 or 31 (and thereby the inlet port). To ensurethat such communication is achieved, the port 43 is partly formed by arecess 43a in the body part 1a and the port 46 is partly formed by arecess 46a in the body part 1a.

The annular recesses 27 and 28 in the piston rods 210 are arranged sothat immediately before and when the piston abuts one or other end ofits cylinder, the annular recess 27 or 28 enters the bore 21b and theport 43 or 46 communicates with the fluid inlet port 32 respectively byway of recess 43a or 460, annular recess 27 or 28, port 30 or 31 andpassage 33 or 34.

The piston lands 22 and 23 are arranged with respect to the ports in thepiston cylinder 3 so that at or towards one end of stroke of the piston,the ports 57 and 60 communicate with the piston transfer chamber 24 andthe ports 54 and 63 communicate with the piston chamber 26; at ortowards the other end of stroke of the piston the ports 54 and 63communicate with the transfer chamber 24 and ports 57 and 60 communicatewith the piston chamber 25; in no position of the piston is either port43 or port 46 in communication with the transfer chamber 24; and in noposition of the piston are both ports 54 and 57 either simultaneouslyclosed by the piston lands 2 and 23 respectively or open to communicatewith the piston transfer chamber 24. r

We will now consider a cycle of operation of the motor above describedand illustrated with reference to FIG. 1. Assume that the motor isassembled in the position drawn, that the inlet port 32 is connected toa source of hydraulic fluid under pressure and that the exhaust port 47is connected to a fluid reservoir.

Passages 32 to 35 and 37 always contain fluid under pressure and, in theposition of the motor illustrated with both the spool and pistonrespectively at and towards one end of their respective strokes, spoolchamber 12a communicates with fluid under pressure by way of port 41,passage 42, port 43 annular recess 27 and port 30 to the passage 33;spool chamber 13a communicates with fluid under pressure by way of port38 and passage 37. Consequently the piston chamber 26 is open to fluidpressure by way of port 46, passage 45 and port 44 which is in permanentcommunication with spool chamber 13a. As the spool 7 is at one endposition of its stroke, ports 52 and 61 are closed by the spool lands 10and 11 respectively but these ports communicateby way of passages 53 and62 with the piston chamber 26 and thereby with fluid under pressure.Spool-biasing chamber 17 communicates with exhaust by way of port 55,passage 56, port 57 and the piston transfer chamber 24 (which is alwaysopen to exhaust) while port 60 can communicate with exhaust by way ofpassage 59 and port 58 which is open to the spool transfer chamber 20(which is always open to exhaust).

As the effective end areas of the spool in the chambers 12a and 13a areequal and both chambers 12a and 13a communicate with fluid underpressure and also as the effective areas of the piston lands 22, 23 inpiston chambers 25, 26 respectively are equal and both piston chambers25 and 26 communicate with fluid under pressure it would appear that themotor is in a stalled condition. However, fluid pressure in the passage53 bleeds into the spool-biasing chamber 15 by way of bleed passage 64and as the spool-biasing chamber 17 is open to exhaust and the combinedeffective area of the spool in the chambers 12a and 15 is larger thanthe effective area of the spool in the chamber 13a the spool is liftedfrom its end seat position as illustrated and commences to move in thedirection of arrow A.

During movement of the spool in the direction of arrow A, control port52 is opened to the spool-biasing chamber 15 to move the spool to theother end of its stroke. By movement of the spool in the direction ofarrow A, port 58 is closed to the spool transfer chamber 20 by spoolland 10 and immediately thereafter the port 36 is opened by the spoolland 8 to spool chamber 12a; port 38 is closed by the spool land 9immediate ly prior to port 61 being opened by the spool land 11 tocommunication with the spool transfer chamber 20. As the control port 52is being opened to the spool-biasing chamber 15 by spool land 10,control port 55 is being closed to the spool-biasing chamber 17 by spoolland 11 and when the spool is located at its end position in thedirection of arrow A, control port 52 is fully open to chamber 15 andcontrol port 55 is closed by land 11.

With the spool at the end of its stroke in the direction of arrow A,port 38 is closed by spool land 9 and consequently spool chamber 13a andpassage 45 are not open to fluid under pressure and the piston chamber26 communicates with exhaust by way of port 63, passage 62 and port 61which is open to the spool transfer chamber 20, and the spool-biasingchamber 15 communicates with exhaust by way of control port 52, passage53 and the piston chamber 26. The piston chamber 25 communicatesdirectly with fluid pressure by way of port 43, passage 42, chamber 12aand port 36 and consequently the piston moves in the direction of arrowA which movement commences when port 61 opens to chamber 20.

During initial movement of the piston 21 in direction of arrow A theport 30 is closed by the full diameter of the piston rod 21a andthereafter the port 60 is closed by the piston land 22 and reopened tothe piston chamber 25 and thereby fluid under pressure (however, sincethe port 58 of passage 59 is closed by the spool land 10 this is withouteffect). Continued movement of the piston causes piston land 22 to closeport 57 and reopen it to the piston chamber 25 and thereby fluidpressure and thereafter causes piston land 23 to close port 54 andreopen it to the piston transfer chamber 24 and thereby exhaust,consequently the passage 56 now contains fluid under pressure and thespool-biasing chamber 15 communicates through port 52, passage 53 andpiston transfer chamber 24 with exhaust. Further movement of the pistoncauses the piston land 23 to close port 63 from the piston chamber 26and reopen it to the piston transfer chamber 24 and thereby exhaust.When port 63 is closed by piston land 23 the piston chamber 26 is closedto exhaust, and the fluid pressure in spool chamber 130, passage 45 andpiston chamber 26 is equalized (by equal and opposite reaction) withfluid pressure in the piston chamber 25 to reverse the spool asdescribed hereafter. The fluid in passage 45 may be subjected to apressure impulse by the inertia of the piston. Since the spool chamber12a is open to fluid pressure (by way of port 36), the spool-biasingchamber 15 is open to exhaust (by way of piston transfer chamber 24) andthe spool-biasing chamber 17 contains fluid under pressure (by way ofthe bleed passage 65 from the passage 56), the fluid pressure in pistonchamber 26 (generated by the piston 21 being arrested at the end of itsstroke in the direction of arrow A together with the pressure appliedthrough the piston from the fluid in piston chamber 25 to equalize thepressure in the piston chambers 25 and 26) causes the spool to move inthe direction of arrow B.

During the movement of the spool 7 in the direction of arrow B, thespool land 11 opens control port 55 to communication with thespool-biasing chamber 17 and the port 61 is closed by the spool land 11immediately prior to the port 38 being opened to the spool chamber 13aby the spool land 9. As port 38 is being opened by spool land 9, port 36is being closed by spool land 8. Port 36 is closed by spool land 8immediately before port 58 is opened to transfer chamber 20 and thespool moves to the end of its stroke in the direction of arrow B asillustrated.

The piston chamber 26 now communicates with fluid pressure by way ofport 46 and passage 45 to chamber 13a while the piston chamber 25communicates with exhaust by way of port 60, passage 59, port 58 and thespool transfer chamber 20 so that the piston reverses and moves in thedirection of arrow B, such movement commencing when port 58 opens tochamber 20. Port 54 communicates with the piston transfer chamber 24(and spool-biasing chamber 15 is consequently open to exhaust by way ofthe bleed passage 64) while the port 57 communicates with the pistonchamber 25 and thereby exhaust.

Continued movement of the piston in the direction of arrow B causespiston land 23 to close port 63 to the piston transfer chamber 24 andreopen it to the piston chamber 26 (which is without effect since theport 61 is closed by the spool land 11). Further movement of the pistoncauses the piston land 23 to close port 54 to the piston transferchamber 24 and reopen it to the piston chamber 26 and thereby fluidunder pressure. Fluid under pressure now bleeds into the spool-biasingchamber by way of the bleed passage 64; however, as the effective areaof the spool face 14 in the chamber 15 is less than the effective areaof the spool end face in the chamber 13a and the piston chamber 25 (andthereby the spool chamber 12a and spool-biasing chamber 17) is open toexhaust, the position of the spool remains unchanged. Still furthermovement of the piston causes the piston land 22 to close the port 57 tothe piston chamber 25 and reopen it to the piston transfer chamber 24and thereby exhaust.

Towards the end of stroke of the piston in the direction of arrow B, thepiston land 22 closes port 60 and thereby the piston chamber 25 iscutoff from exhaust. As soon as the port 60 is closed by the piston land22 from the chamber 25, the fluid pressure in the chamber 25, passage 42and spool chamber 12a acts to reverse the spool as described hereafter.The pressure in piston chamber 25 is equalized to the fluid pressure inthe chamber 26 (by equal and opposite reaction through the piston 21)and consequently the resultant pressure on the spool 7 in the spoolchamber 12a and the spoolbiasing chamber 15 (together with the pressureimpulse in spool chamber 12a developed by the loss in inertia of thepiston) overcomes the opposing pressure on the spool in the chamber 13aand the spool is lifted from its end position and moved in the directionof arrow A and the motor commences a further cycle of reciprocation.

It is possible that during operation or assembly of the motor acondition can occur in which the piston is located at an extreme end ofits stroke (i.e., when one of the piston lands 22, 23, abuts an end ofthe piston cylinder and the associated port 60, 63 is closed to itsassociated piston chamber 25, 26, then it will be apparent that fluidpressure in the piston chambers 25 and 26 cannot be equalized. In such acondition, although one or other of the spool-biasing chambers 15, 17will contain fluid under pressure and act in a sense to reverse thespool, if both the spool chambers 12a and 130 do not contain fluid atthe same pressure then the spool will not reverse and the motor willstall. To ensure that the motor will not stall with the piston at anextreme end of its stroke the passages 33 and 34 and respective ports 30and 31 are provided which are adapted to communicate with the annularrecesses 27 and 28 in the piston rod 21a when the piston is at ortowards one or other extreme ends of its stroke (that is after one orother of the ports 60, 63 has been closed to its respective pistonchamber 25, 26). If a condition now arises in which the piston is at anextreme end of its stroke, say in the direction of arrow B in thedrawing, fluid under pressure in the passage 33 can now communicatethrough port 30, annular recess 27, recess 43a of port 43 and passage 42with the chamber 12a so that the fluid pressure in chambers 12a and 13ais equalized and the spool is moved in the direction of arrow A by theeffect of fluid pressure in spool-biasing chamber 15. Similarly, whenthe piston is located at the extreme end of its stroke in the directionof arrow A, fluid under pressure passes through the port 31, annularrecess 28, recess 46a of port 46 and passage 45 to the chamber 13a toequalize pressure in the chambers 12a and 134 so that fluid pressure inthe spool-biasing chamber 17 causes the spool to reverse and move in thedirection of arrow B.

The cooperation between the annular recesses 27, 28, in the piston rodand associated ports 30, 31, effectively constitute valves and can bereplaced by alternative types of valves as will be apparent to personsskilled in the art and which are automatically actuated to equalizefluid pressure in the spoolworking chambers 12a, 13a in the event of thepiston 21 coming to rest at, or being assembled in, one or other extremeends of its stroke (when the piston land 22 or 23 abuts the end of thepiston cylinder).

The spool lands 8 to 11 are conveniently provided with annular recessesto 11a respectively which recesses are located so that, when theirrespective land closes a port containing fluid pressure, the port inquestion communicates with the recess and fluid pressure on the spoolfrom the port is equalized around the spool thereby maintaining thespool in balance in its cylinder. In no position of the spool do any ofthe annular recesses 80 to lla bridge two or more ports in the spoolcylinder.

In the above described embodiment illustrated with reference to FIG. 1,it is seen that for reversal of the spool 7 from one end of its stroketo the other it is necessary for fluid under pressure to be present ineither the spool-biasing chamber 15 or chamber 17 and for the fluidpressure in chambers 12a and 13a to be equalized. As described, fluidunder pressure can enter the spool-biasing chambers 15 and 17 either bycontrolled seepage from the ports 52 and 55 respectively or by way ofbleed passages 64 and 65 respectively. By so pressurizing thespool-biasing chambers it is possible, when the piston is reciprocatingat high speed, for a back pressure to build up in one or other of thespool chambers 12a,13a which back pressure, together with pressure inthe adjacent spoolbiasing chamber, could result in the spool reversingprematurely and thereby cause a short stroke for the piston. Forexample, with the piston moving at high speed in the direction of arrowB and the spool in the position shown in FIG. 1, with the passage 53(and thereby the spool-biasing chamber 15 by way of passage 64)communicating with fluid pressure in piston chamber 26 it is possible,even with the piston chamber 25 open to exhaust by way of port 60,passage 59, and spool transfer chamber 20, for back pressure to build upin chamber 12a by way of passage 42 and port 43. The combined forceresulting from the back pressure and from pressure in spoolbiasingchamber 15 could be greater than the force resulting from the pressurein spool chamber 13a and in such case the spool would reverseprematurely and thereby cause the piston to reverse prematurely. Suchpremature reversal of the spool is unlikely to occur when the piston isreciprocating at slow speeds but it is possible during high-speedreciprocation of the piston and the piston could, as a consequence,exhibit short stroking. The possibility of such short stroking of thepiston is generally undesirable and a preferred embodiment of thepresent invention will now be described with reference to FIG. 2, inwhich the possibility of premature reversal of the spool and therebyshort stroking of the piston is minimized.

For convenience of description, the same parts or members of the motorillustrated in FIG. 2 to those above described and illustrated withreference to FIG. 1 have been accorded the same references.

In FIG. 2 the bleed passages 64, 65 are omitted and the spool-biasingchambers are not intended to be pressurized either by controlled seepageor otherwise directly from the passages 53, 56. Similarly the ports 30and 31 and their respective passages 33 and 34 are omitted and the endsof the piston cylinder 3 are recessed at 66 so that the port 43 is inconstant communication with piston chamber 25 and the port 46 is inconstant communication with piston chamber 26 even when the piston islocated at one or other extreme end of its stroke (that is when pistonland 22 or 23 abuts the adjacent end of the piston cylinder).

Located in the spool cylinder part 4 is a signalling port 67 whichcommunicates with the spool-biasing chamber 15 when the spool is locatedat the end of its stroke in the direction of arrow B and similarlylocated in the spool cylinder part 40 is a further signalling port 68which communicates with the spoolbiasing chamber 17 when the spool islocated at the end of its stroke in the direction of arrow A. The port67 communicates with the chamber 15 by way of an annular recess 69 inthe spool 7 and the port 68 communicates with the chamber 17 by way ofan annular recess 70 in the spool 7. The ports 67, 68 and recesses 69,70 are so located with respect to the ports 52 and 55 that, when thespool moves rightwardly from the end of its stroke and in the directionof arrow A, the signalling port 67 maintains communication with thespool-biasing chamber until the port 52 is opened by the spool land 10to communication with the spool-biasing chamber 15, after which thesignalling port 67 is closed by the spool land 8 and, when the spoolmoves leftwardly from the end of its stroke and in the direction ofarrow B, the signalling port 68 maintains communication with thespool-biasing chamber 17 until the port 55 is opened by the spool land11 tocommunication with the spoolbiasing chamber 17, after which thesignalling port 68 is closed by the spool land 9.

The signalling port 67 communicates through a passage 71 with a port 72in the bore 21b of the body part 34 1a at the end of the piston cylinderadjacent the port 43. Similarly the signalling port 68 communicatesthrough a passage 73 with a port 74 in the bore 21b of the body part laat the end of the piston cylinder adjacent the port 46. Located in thebore 21b diametrically opposite (or in the same diametral plane as) theport 72 is a port 75 which communicates by way of a passage 76 with thepassage 53 and thereby with the ports 52 and 54. Similarly located inthe bore 21b diametrically opposite (or in the same diametral plane as)the port 74 is a port 77 which communicates by way of a passage 78 withthe passage 56 and thereby with the ports 55 and 57.

The ports 72 and 75 are so positioned in the bore 21b that, as thepiston 21 moves in the direction of arrow B, they are caused tocommunicate with each other by way of the annular recess 27 in thepiston rod 21a immediately before the port 60 is closed by the pistonland 22 and thereafter are maintained in communication through therecess 27 until the piston land 22 abuts the end of the piston cylinder3 at the end of stroke of the piston in the direction of arrow B.

As the piston land 22 approaches, and immediately before it abuts, theend of the piston cylinder, a normally closed small port 79 in the bore21b is opened to the recess 27 so that at the limit of stroke of thepiston in the direction of arrow B the ports 72, 75, and 79 are incommunication by way of recess 27. Similarly the ports 74 and 77 are sopositioned in the bore 21b that, as the piston 21 moves in the directionof arrow A, they are caused to communicate with each other by way of theannular recess 28 in the piston rod 21a immediately before the port 63is closed by the piston land 23 and thereafter are maintained incommunication through the recess 28 until .the

piston land 23 abuts the end of the piston cylinder 3. As the pistonland 23 approaches, and immediately before it abuts the end of thepiston cylinder, a normally closed small port 81 in the bore 21b isopened to the recess 28 so that at the limit of stroke of the piston inthe direction of arrow A, the ports 74, 77 and 81 are in communicationby way of recess 28.

The port 79 in the bore 21b is in permanent communication with thepiston chamber 25 by way of a passage 80 and similarly the port 81 is inpermanent communication with the piston chamber 26 by way of a passage82.

It is to be noted in FIG. 2 that the annular recesses 27 and 28 areaxially spaced from their adjacent piston lands and are so arranged thatrecess 27 cannot communicate directly between the ports 72, 75, and thepiston chamber 25, and recess 28 cannot communicate directly between theports 74, 77 and the piston chamber 26.

We will now consider a cycle of operation of the motor above describedand illustrated with reference to FIG. 2. Assume that the motor isassembled in the position drawn, that the inlet port 32 is connected toa source of hydraulic fluid under pressure and that the exhaust port 47is connected to a fluid reservoir.

Passages 32, 35 and 37 always contain fluid under pressure and, in theposition of the motor illustrated with both the spool and pistonrespectively at, and approaching one end of their respective strokes,spool chamber 13a communicates with fluid under pressure by way of port38 and the spool chamber 12a communicates with exhaust by way of port41, passage 42, piston chamber 25, port 60, passage 59, spool transferchamber and passage 47 so that the spool is held at the end of itsstroke in the direction of arrow B, while the piston chamber 26communicates with fluid pressure by way of passage 45 and spool chamber13a and the piston chamber 25 communicates with exhaust by way of port60, passage 59, spool transfer chamber 20 and passage 47 so that thepiston is moving in the direction of arrow B.

The piston is shown with its land 23 opening port 54 to communicationwith piston chamber 26 (which is without effect since the ports 52 and75 are closed by spool land 10 and piston rod 210 respectively). Furthermovement of the piston causes the piston land 22 to close the port 57 tothe piston chamber 25 and reopen it to the piston transfer chamber 24and thereby exhaust.

Towards the end of stroke of the piston in the direction of arrow B, theports 72 and 75 are placed in communication through the piston recess 27immediately before the piston land 22 closes port 60 and consequentlythe spool-biasing chamber 15 communicates with fluid pressure by way ofsignalling port 67, passage 71, recess 27, port 75, passages 76 and 53,port 54 and piston chamber 26. However, since the effective area of thespool face 14 in the spool-biasing chamber 15 is less than the effectivearea of the spool end face in the spool-working chamber 13a and thepiston chamber 25 (and thereby the spool-biasing chamber is open toexhaust, the position of the spool remains unchanged. As soon as theport 60 is closed by the piston land 22 from the chamber 25, the chamber25 is cut off from exhaust. Fluid pressure in piston chamber 25, passage42 and spool chamber 12a is equalized to the fluid pressure in thepiston chamber 26 and spool chamber 13a and consequently the resultantpressure on the spool 7 in the spool chamber 120 and the spool biasingchamber 15 (by way of port 67) overcomes the opposing pressure on thespool in the working chamber 13a and the spool is lifted from its endposition and moved in the direction of arrow A. On completion ofreversal of the spool the piston 21 is hydraulically arrested.

During movement of the spool in the direction of arrow A, control port52 is opened to the spool-biasing chamber 15, (prior to the signallingport 67 being closed by spool land 8) to move the spool to the other endof its stroke. By movement of the spool in the direction of arrow A,port 58 is closed to the spool transfer chamber 20 by spool land 10 andimmediately thereafter the port 36 is opened by the spool land 8 to thespool-working chamber 12a; port 38 is closed by the spool land 9immediately prior to port 61 being opened by the spool land 11 tocommunication with the spool transfer chamber 20 and signalling port 68being opened to communication with the spool-biasing chamber 17 by wayof spool recess 70. The ports 38 and 61 are located with respect to thespool lands 9 and 11 and the ports 36 and 52 are located with respect tothe spool lands 8 and 10 so that in no position of the spool are bothports 38 and 61 simultaneously open from the spool lands 9 and 11respectively and in no position of the spool are both ports 36 and 58simultaneously open from the spool lands 8 and 10 respectively. As thecontrol port 52 is being opened to the spool-biasing chamber 15 by spoolland 10, control port 55 is being closed to the spool-biasing chamber 17by spool land 11 and when the spool is located at its endof strokeposition in the direction of arrow A control port 52 is fully open andcontrol port 55 is closed.

With the spool at the end of its stroke in the direction of arrow A,port 38 is closed by spool land 9 and consequently spool-working chamber13a and passage 45 are not open to fluid under pressure and the pistonchamber 26 communicates with exhaust by way of port 63, passage 62 andport 61 which is open to the spool transfer chamber 20. Thespool-biasing chamber 15 communicates with exhaust by way of controlport 52, passage 53 and the piston chamber 26. The piston chamber 25communicates with fluid pressure by way of port 43, passage 42, chamber120 and port 36 and consequently the piston moves in the direction ofarrow A.

During initial movement of the piston 21 in direction of arrow A theport 60 is closed by the piston land 22 from piston transfer chamber 24and reopened to the piston chamber 25 and thereby fluid under pressure(however, since the port 58 passage 76 communicate with exhaust by wayof through port 52, passage 53 and piston transfer chamber 24. Furthermovement of the piston causes the piston land 23 to close port 63 fromthe piston chamber 26 and reopen it to the piston transfer chamber 24and thereby exhaust. However, immediately before the piston land 23closes port 63, the ports 74 and 77 are placed in communication throughthe piston recess 28 and consequently the spool-biasing chamber 17communicates with fluid pressure by way of signalling port 68, passage73, port 74, recess 28, passages 78 and 56, port 57 and piston chamber25. When port 63 is closed by piston land 23, fluid pressure developedin spool-working chamber 131:, passage 45 and piston chamber 26 acts toreverse the spool. The fluid in passage 45 is subjected to a pressureimpulse (partly by the inertia of the piston). Since the spool-workingchamber 12a is open to fluid pressure (by way of port 36), thespool-biasing chamber is open to exhaust (by way of piston transferchamber 24) and the spool-biasing chamber 17 contains fluid underpressure (by way of passages 73, 78, and 56), the pressure appliedthrough the piston from fluid pressure in piston chamber 25 to equalizethe pressures in the piston chambers 25 and 26 and also equalize thepressures in spool chambers 12a and 13a causes (by fluid pressure in thespoolbiasing chamber 17) the spool to move in the direction of arrow B.On completion of reversal of the spool, the movement of the piston atthe end of its stroke in the direction of the arrow A is arrested.

During movement of the spool 7 in the direction ofarrow B, the spoolland 11 opens control port 55 to communication with the spool-biasingchamber 17 immediately before the signalling port 68"is closed by spoolland 9 and the port 61 is closed by the spool land 11 immediately priorto the port 38 being opened to the spool-working chamber 13a by thespool land 9. As port 38 is being opened by spool land 9, port 36 isbeing closed by spool land 8 until the spool reaches the end of itsstroke in the direction of arrow B as illustrated in H0. 2.

The piston chamber 26 now communicates with fluid pressure by way ofport 46 and passage 45 to chamber 13a while the piston chamber 25communicates with exhaust by way of port 60, passage 59, port 58 and thespool transfer chamber so that the piston commences to move in thedirection of arrow B. Port 54 communicates with the piston transferchamber 24 and thereby exhaust while the port 57 communicates with thepiston chamber 25 and thereby exhaust.

Further movement of the piston in the direction of arrow B causes thepiston land 23 to close port 54 to piston transfer chamber 24 and reopenit to piston chamber 26 and thereby fluid under pressure is applied tothe passages 53 and 76. Continued movement of the piston causes thepiston land 22 to close port 57 to the piston chamber 25 and reopen itto the piston transfer chamber 24 which maintains passages 56 and 78 incommunication with exhaust and the motor commences a further cycle ofoperation.

From the aforegoing description with reference to FIG. 2, it will beseen that the alternate communication of the spoolbiasing chambers 15and 17 with fluid pressure is closely controlled by the position of thepiston in its cylinder and that each spool-biasing chamber is onlypressurized when the piston is located closely adjacent to itsrespective end of stroke position. Consequently the possibility of thepiston exhibiting short stroking due to premature reversal of the spoolis minimized. In effect the interaction between the ports 72, 75 andpiston recess 27 (and similarly between the ports 74, 77 and pistonrecess 28) constitute an auxiliary valve which controls the timing atwhich the spool-biasing chamber 15 (and 17) is pressurized in readinessfor the spool to reverse and as such may be replaced by other forms ofvalve means as will be apparent to persons skilled in the art (forexample electrically controlled valves) which are responsive to theposition of the piston in its cylinder so that the spool-biasing chamber15 is automatically pressurized immediately prior to port 60 beingclosed during movement of the piston in the direction of arrow B and thespool-biasing chamber 17 is automatically pressurized immediately priorto port 63 being closed during movement of the piston in the directionof arrow A.

If a condition ever occurs during operation or assembly of the motor inwhich the piston is located at an extreme end of its stroke (i.e., whenone of the pistonlands 22, 23 abuts an end of the piston cylinder 3),then it will be apparent that fluid pressure in the piston chambers 25and 26 cannot be equalized by equal and opposite reaction through thepiston and in such a condition, although'one or othei of thespool-biasing chambers l5, 17 will contain fluid under pressure, if boththe spool-working chambers 12a and 13a do not contain fluid at the samepressure then the spool will not reverse and the motor will stall. Toensure that such a condition cannot occur, the passages and 82 andrespective ports 79 and 81 are provided which are adapted to communicatewith the annular recesses 27 and 28 in the piston rod 21a and throughrecesses 27 and 28 with ports 72, 75 and 74, 77 respectively when thepiston is at or towards one or other extreme end of its stroke (that isafter one or other ports 60, 63 has been closed to its respective pistonchamber 25, 26 and the respective spoolbiasing chamber 16, 17) have beenopened to fluid pressure). If the condition now arises in which thepiston is at an extreme end of its stroke, say in the direction of arrowB in the drawing, fluid under pressure in the passage,7l6 can nowcommunicate through port 75, recess 27, port 79, passage 80, recess 66,port 43 and passage 42 with the chamber 12a so that fluid pressure inspool-working chambers 12d and 13a is equalized and the spool is movedin the direction bf arrow A by the effect of fluid pressure inspool-biasing chamber 15. Similarly, when the piston is located at theextreme end of its stroke in the direction of arrow A, fluid underpressure in the passage 78 can now communicate through port 77, recess28, port 81, passage 82, recess 66, port 46 and passage 45 to thechamber 13a to equalize fluid pressure in the spool-working chambers 12aand 13a so that fluid pressure in the spool-biasing chamber 17 causesthe spool to reverse and move in the direction of arrow B.

The cooperation between the annulai' recesses 27, 28 and associatedports 75, 79, 77, 81 effectively constitute valves and can be replacedby alternative forms of valve means as will be apparent to personsskilled in the art and which are automatically actuated to equalizefluid pressure in the spoolworking chambers 12a, 13a in the event of thepiston 21 coming to rest at, or being assembled in, one or other extremeends of its stroke.

in fluid pressure operated motors, a condition in which stalling is mostlikely to occur is that in which the piston and/or spool are located intheir respective cylinders at the center of oscillation. in each of theabove described embodiments assume that the piston 21 is located so thatport 57 communicates with piston chamber 25 and port 54 communicateswith piston chamber 26 (i.e., ports 54 and 57 are partly obturated bypiston lands 23 and 22 respectively as shown in FIG. 2); that the spool7 is located so that port 52 communicates with the spool-biasing chamber15 and port 55 communicates with the spool-biasing chamber 17; thatports 36 and 38 are partly open to their respective spool-workingchambers 12a, 13a and that ports 58 and 61 are closed by the spool lands10 and 11 respectively. In such a condition the piston chamber 26contains fluid under pressure by way of spool chamber 13a and passage 55and consequently spoolbiasing chamber 15 contains fluid under pressure,while spoolbiasing chamber 17 communicates with the piston chamber 25 byway of port 55, passage 56 and partly open port 57 as does spool chamber12a (which is open to fluid pressure) by way of passage 42 and port 43.Consequently the pressure of fluid in the chambers 25, 26, l5, 17, 12aand 13a is equalized. However, since the areas of the spool end faces inchambers 12a and 13a are equal and the effective area on the spool sideface 16 is slightly larger than the effective area on the spool sideface 14, the spool is pressure biased and moves in the direction ofarrow B to the end of its stroke and a stalled condition of the motor isthereby avoided.

In fluid pressure operated motors of the type in which reciprocation ofa piston and spool mutually control the timing of reciprocation andreversal of each other, movement of the piston in its cylinder serves toactuate (or comprises) a valve by which fluid pressure has generallyhitherto been directed from the fluid pressure source (usually a pump)to reverse the spool. By so doing part of the pressure output or volumefrom the pump is diverted from driving the piston to reversing the spooland consequently the piston tends to exhibit an appreciable decrease inits axial speed towards the ends of its stroke and possibly anappreciable reversal time. By a motor constructed in accordance with thepresent invention in which reversal of the spool is effected by fluidpressure in both spool-working chambers being caused to apply equal andopposite axial thrusts on the spool when the port (60 or 63) throughwhich the contracting piston chamber exhaustsis closed (by the pistonland 22 or 23), the total output of the pump may be used to drive thepiston for substantially the whole length of its stroke since no fluidpressure need be diverted from the pump to reverse the spool.Consequently the motor of the present invention may have minimalreversal time for its piston without appreciable slowing down of thepiston towards the ends of its stroke. In the drawings (see FIG. 2) itwould appear that, during reversal of the spool rightwardly from theposition shown, fluid output is diverted from the pump or pressure inlet32 by way of piston chamber 26, passages 53, 76 and 71, and control andsignalling ports 52 and 67 into the spool-biasing chamber 15 to effectreversal of the spool. However, it must be borne in mind that thespoolworking chamber 13a is contracting and the volume of fluiddisplaced therefrom may be arranged to be fractionally more than thatentering the spool-biasing chamber 15 so that practically the totalvolume output from the pump is available for driving the piston.

lclaim:

l. A fluid-pressure-operated motor reciprocating motion which includes aspool axially slidable in a spool cylinder, the spool having a firstpair of opposed faces each of which respectively communicates with oneof a pair of spool-working chambers in the spool cylinder, and a secondpair of opposed faces each of which respectively communicates with oneof a pair of spool-biasing chambers in the spool cylinder;

a piston axially slidable in a piston cylinder, the piston being doubleacting and having a pair of opposed faces each of which respectivelycommunicates with one of a pair of piston chambers, reversal of thepiston being initiated by reversal of the spool which serves to connectone piston chamber alternately to fluid pressure and to exhaust and toconnect the other piston chamber alternately to exhaust and to fluidpressure respectively;

and wherein the timing of reversal of the spool is determined accordingto the position of the piston in its cylinder, the arrangement beingsuch that with the spool at an end of its stroke, one spool-workingchamber is open to fluid pressure while the other spool-working chamberis open to exhaust to urge the spool to that end and one spool-biasingchamber is open to exhaust and, to reverse the spool, fluid pressure inboth spool-working chambers is caused to apply equal and opposite axialthrusts on the spool and fluid pressure is caused to be admitted to theother spool-biasing chamber in opposition to the load on the spoolapplied by fluid pressure in said one spool-working chamber.

for producing 2. A motor as claimed in claim 1 in which the first pairof opposed faces of the spool are of equal effective area, the secondpair of opposed faces of the spool each have an effective area less thanthe effective area of one of the first pair of opposed faces, and thepair of opposed faces of the piston are of equal effective area andwherein the arrangement is such that, to reverse the spool, fluidpressure is caused to be admitted to the other spool-biasing chamber inopposition to the load on the spool applied by fluid pressure in saidone spoolworking chamber and fluid pressure in both spool-workingchambers is caused to be equalized.

3. A motor as claimed in claim 1 in which the arrangement is such thatduring axial movement of the piston fluid pressure is caused to beadmitted to the said other spool-biasing chamber and thereafter, at ortowards the end of stroke of the piston consistent with its axialmovement, fluid pressure in both spool-working chambersis caused toapply equal and opposite axial thrusts on the spool to reverse thespool. v

4. A motor as claimed in claim 1 in which the second pair of opposedfaces of the spool are of different effective area.

5. A motor as claimed in claim 1 in which each of the pair of pistonchambers is respectively in constant communication with one of the pairof spool-working chambers and each piston chamber has associatedtherewith a port through which it is adapted to be exhausted andwherein, during axial movement of the piston, the port through which thecontracting piston chamber is exhausting is adapted to be closed by thepiston towards its end of stroke and thereafter fluid pressure in thecontracting piston chamber and spool working chamber in communicationtherewith applies an axial thrust to the spool which equalizes theopposing axial thrust applied to the spool by fluid pressure in theexpanding piston chamber and spoolworking chamber in communicationtherewith.

6. A motor as claimed in claim 5 wherein fluid under pressure isdelivered into the expanding piston chamber from the spool-workingchamber in communication therewith and the spool-working chambers areadapted respectively to communicate with fluid pressure by way, of oneof a pair of fluid inlet ports controlled by the spool to be alternatelyopened and closed to their respective spool-working chambers so thatwith the spool at an end of its stroke the expanding piston chambercommunicates with the spool-working chamber which is open to fluidpressure and the contracting piston chamber communicates with thespool-working chamber which is closed to fluid pressure.

7. A motor as claimed in claim 6 wherein the pair of fluid inlet portsare so controlled by the spool that in no position of the spool are theysimultaneously closed to their respective spool-working chambers.

8. A motor as claimed in claim 5 in which two ports are provided in thepiston cylinder through one of which ports each piston chamber isrespectively adapted to be exhausted, said two ports communicatingrespectively with two ports in the spool cylinder which are alternatelyopened and closed to communication with exhaust by reciprocation of thespool, and wherein said two ports in the spool cylinder are so arrangedand controlled by the spool that in no position of the spool are saidports simultaneously open to exhaust and, with the spool at an end ofstroke position, the port in the spool cylinder which communicates withthe expanding piston chamber is closed by the spool to exhaust and theport in the spool cylinder which communicates with the contractingpiston chamber is open to exhaust and, on reversal of the spool, theport in the spool cylinder which communicates with exhaust is adapted tobe closed by the spool prior to fluid pressure being admitted to itsrespective piston chamber and the port in the spool cylinder which isclosed to exhaust is adapted to be opened by the spool to communicationwith exhaust subsequent to fluid pressure being cut off from itsrespective piston chamber.

9. A motor as claimed in claim 1 wherein two control ports are providedin the spool cylinder each of which is adapted to respectivelycommunicate with one of the pair of spool-biasing chambers, the controlports being controlled by reciprocation of the spool to be alternatelyopened and closed to their respective spool-biasing chambers and withthe spool at an end of its stroke one control port is open and the othercontrol port is closed to its respective spool-biasing chamber, theclosed control port being associated with the spool-biasing chamberwhich, if subjected to fluid pressure, would urge the spool away fromits said end of stroke position; said control ports being adapted tocommunicate alternately with fluid pressure and exhaust duringreciprocation of the piston so that, with the spool at an end of itsstroke, when the piston reaches an end of its stroke consistent with thespool in said end of stroke position the closed control portcommunicates with fluid pressure and the open control port and therebyits respective spool-biasing chamber communicates with exhaust andreversal of the spool causes the spool to open the control port incommunication with fluid pressure to its respective spool-biasingchamber to urge the spool to its other end of stroke position.

10. A motor as claimed in claim 9 wherein the two control ports are solocated and controlled by the spool that in no position of the spool arethey simultaneously closed to their respective spool-biasing chambers.

11. A motor as claimed in claim 9 wherein each of the two control portsrespectively communicates with one of two ports in the piston cylinder,each of the two ports in the piston cylinder being adapted torespectively communicate with one of the pair of piston chambers so thatwith the spool at an end of stroke position the control port which isclosed by the spool communicates with the expanding pressure chamber andthe control port which is open to its associated spool-biasing chambercommunicates with exhaust when the piston reaches its end of strokeposition.

12. A motor as claimed in claim 11 wherein the two ports in the pistoncylinder which communicate with the control ports are so located andcontrolled by the piston that in no position of the piston are theysimultaneously closed to communication with their respectivelyassociated piston chambers.

13. A motor as claimed in claim 11 in which the piston defines with itscylinder a piston transfer chamber located axially between the pair ofpiston chambers and wherein said piston transfer chamber is in constantcommunication with exhaust and thetwo ports in the piston cylinder whichcommunicate with the control ports are so located and controlled by thepiston that during reciprocation of the piston each such portalternately communicates with its respectively associated piston chamberand the piston transfer chamber.

14. A motor as claimed in claim 13 in which the two ports in the pistoncylinder which communicate with the control ports are so located andcontrolled by the piston that in no position of the piston are theysimultaneously open to communication with the piston transfer chamber.

15. A motor as claimed in claim 1 in which fluid pressure is caused tobe admitted to the other spool-biasing chamber by way of valve meansresponsive to axial movement of the piston.

16. A motor as claimed in claim 15 wherein said valve means is adaptedto be actuated to admit fluid pressure to the Said other spool-biasingchamber immediately before the port through which the contracting pistonchamber exhausts is closed by the piston.

17. A motor as claimed in claim 15 in which the valve means comprisescooperating ports and lands between the piston and its cylinder.

18. A motor as claimed in claim 9 in which fluid pressure is caused tobe admitted to the other spool-biasing chamber by way of valve meansresponsive to axial movement of the piston, and in which the spoolcylinder has a pair of signalling ports each of which is adapted torespectively communicate with one of the pair of spool-biasing chambers,the signalling ports being controlled by reciprocation of the spool tobe alternately opened and closed to their respective spool-biasingchambers and with the spool at an end of its stroke one signalling portis open and the other signalling port is closed to its respectivespool-biasing chamber, the open signalling port being associated withthe spool-biasing chamber which, if subjected to fluid pressure, wouldurge the spool away from its said end of stroke position; saidsignalling ports being adapted to communicate with fluid pressure andbeing controlled for such communication by way of said valve means; andwherein for each spool-biasing chamber, the signalling port and controlport respectively associated therewith are so located and controlled bythe spool that in no position of the spool are they simultaneouslyclosed to their respective spool-biasing chamber.

19. A motor as claimed in claim 18, wherein for each spoolbiasingchamber, the signalling port associated therewith is adapted tocommunicate by way of said valve means with fluid pressure at thecontrol port associated therewith.

20. A motor as claimed in claim I in which fluid under pressure iscaused to be admitted to the other spool-biasing chamber by way of bleedpassage means which is in constant communication with said otherspool-biasing chamber and is adapted to communicate with fluid pressureduring expansion of a piston chamber consistent with the spool at saidend and to communicate with exhaust during contraction of a pistonchamber consistent with the spool at the other end of its stroke.

21. A motor as claimed in claim 9 in which fluid under pressure iscaused to be admitted to the other spool-biasing chamber by way of bleedpassage means which is in constant communication with said otherspool-biasing chamber and is adapted to communicate with fluid pressureduring expansion of a piston chamber consistent with the spool at saidend and to communicate with exhaust during contraction of a pistonchamber consistent with the spool at the other end of its stroke, and inwhich each spool-biasing chamber and control port respectivelyassociated therewith are in constant communication by way of a bleedpassage.

22. A motor as claimed in claim 8 in which fluid under pressure iscaused to be admitted to the other spool-biasing chamber by way of bleedpassage means which is in constant communication with said otherspool-biasing chamber and is adapted to communicate with fluid pressureduring expansion of a piston chamber consistent with the spool at saidend and to communicate with exhaust during contraction of a pistonchamber consistent with the spool at the other end of its stroke, and inwhich each spool-biasing chamber and control port respectivelyassociated therewith are in constant communication by way of a bleedpassage.

23. A motor as claimed in claim 1 wherein auxiliary valve means isprovided which is adapted to be actuated when the piston is located ator towards an end of stroke position and, with the spool at an end ofstroke position, actuation of said auxiliary valve means admits fluidpressure to the spool-working chamber which, when pressurized urges thespool to its other end of stroke position.

24. A motor as claimed in claim 23 in which means is provided by whichsaid auxiliary valve means cannot be actuated to admit fluid pressure tosaid spool-working chamber while said spool-working chamber communicateswith exhaust.

25. A motor as claimed in claim 5 wherein auxiliary valve means isprovided which is adapted to be actuated when the piston is located ator towards an end of stroke position and, with the spool at an end ofstroke position, actuation of said auxiliary valve means admits fluidpressure to the spool-working chamber which, when pressurized urges thespool to its other end of stroke position, and in which, during axialmovement of the piston, the auxiliary valve means is adapted to beactuated to admit fluid pressure to said spool-working chamber only whenthe piston is located at a position in which the port through which thecontracting piston chamber exhausts is closed by the piston fromcommunication with said piston chamber.

26. A motor as claimed in claim 25 in which the auxiliary valve means isadapted to admit fluid pressure into the con-

1. A fluid-pressure-operated motor for producing reciprocating motionwhich includes a spool axially slidable in a spool cylinder, the spoolhaving a first pair of opposed faces each of which respectivelycommunicates with one of a pair of spool-working chambers in the spoolcylinder, and a second pair of opposed faces each of which respectivelycommunicates with one of a pair of spoolbiasing chambers in the spoolcylinder; a piston axially slidable in a piston cylinder, the pistonbeing double acting and having a pair of opposed faces each of whichrespectively communicates with one of a pair of piston chambers,reversal of the piston being initiated by reversal of the spool whichserves to connect one piston chamber alternately to fluid pressure andto exhaust and to connect the other piston chamber alternately toexhaust and to fluid pressure respectively; and wherein the timing ofreversal of the spool is determined according to the position of thepiston in its cylinder, the arrangement being such that with the spoolat an end of its stroke, one spool-working chamber is open to fluidpressure while the other spool-working chamber is open to exhaust tourge the spool to that end and one spool-biasing chamber is open toexhaust and, to reverse the spool, fluid pressure in both spool-workingchambers is caused to apply equal and opposite axial thrusts on thespool and fluid pressure is caused to be admitted to the otherspool-biasing chamber in opposition to the load on the spool applied byfluid pressure in said one spool-working chamber.
 2. A motor as claimedin claim 1 in which the first pair of opposed faces of the spool are ofequal effective area, the second pair of opposed faces of the spool eachhave an effective area less than the effective area of one of the firstpair of opposed faces, and the pair of opposed faces of the piston areof equal effective area and wherein the arrangement is such that, toreverse the spool, fluid pressure is caused to be admitted to the otherspool-biasing chamber in opposition to the load on the spool applied byfluid pressure in said one spool-working chamber and fluid pressure inboth spool-working chambers is caused to be equalized.
 3. A motor asclaimed in claim 1 in which the arrangement is such that during axialmovement of the piston fluid pressure is caused to be admitted to thesaid other spool-biasing chamber and thereafter, at or towards the endof stroke of the piston consistent with its axial movement, fluidpressure in both spool-working chambers is caused to apply equal andopposite axial thrusts on the spool to reverse the Spool.
 4. A motor asclaimed in claim 1 in which the second pair of opposed faces of thespool are of different effective area.
 5. A motor as claimed in claim 1in which each of the pair of piston chambers is respectively in constantcommunication with one of the pair of spool-working chambers and eachpiston chamber has associated therewith a port through which it isadapted to be exhausted and wherein, during axial movement of thepiston, the port through which the contracting piston chamber isexhausting is adapted to be closed by the piston towards its end ofstroke and thereafter fluid pressure in the contracting piston chamberand spool working chamber in communication therewith applies an axialthrust to the spool which equalizes the opposing axial thrust applied tothe spool by fluid pressure in the expanding piston chamber andspool-working chamber in communication therewith.
 6. A motor as claimedin claim 5 wherein fluid under pressure is delivered into the expandingpiston chamber from the spool-working chamber in communication therewithand the spool-working chambers are adapted respectively to communicatewith fluid pressure by way of one of a pair of fluid inlet portscontrolled by the spool to be alternately opened and closed to theirrespective spool-working chambers so that with the spool at an end ofits stroke the expanding piston chamber communicates with thespool-working chamber which is open to fluid pressure and thecontracting piston chamber communicates with the spool-working chamberwhich is closed to fluid pressure.
 7. A motor as claimed in claim 6wherein the pair of fluid inlet ports are so controlled by the spoolthat in no position of the spool are they simultaneously closed to theirrespective spool-working chambers.
 8. A motor as claimed in claim 5 inwhich two ports are provided in the piston cylinder through one of whichports each piston chamber is respectively adapted to be exhausted, saidtwo ports communicating respectively with two ports in the spoolcylinder which are alternately opened and closed to communication withexhaust by reciprocation of the spool, and wherein said two ports in thespool cylinder are so arranged and controlled by the spool that in noposition of the spool are said ports simultaneously open to exhaust and,with the spool at an end of stroke position, the port in the spoolcylinder which communicates with the expanding piston chamber is closedby the spool to exhaust and the port in the spool cylinder whichcommunicates with the contracting piston chamber is open to exhaust and,on reversal of the spool, the port in the spool cylinder whichcommunicates with exhaust is adapted to be closed by the spool prior tofluid pressure being admitted to its respective piston chamber and theport in the spool cylinder which is closed to exhaust is adapted to beopened by the spool to communication with exhaust subsequent to fluidpressure being cut off from its respective piston chamber.
 9. A motor asclaimed in claim 1 wherein two control ports are provided in the spoolcylinder each of which is adapted to respectively communicate with oneof the pair of spool-biasing chambers, the control ports beingcontrolled by reciprocation of the spool to be alternately opened andclosed to their respective spool-biasing chambers and with the spool atan end of its stroke one control port is open and the other control portis closed to its respective spool-biasing chamber, the closed controlport being associated with the spool-biasing chamber which, if subjectedto fluid pressure, would urge the spool away from its said end of strokeposition; said control ports being adapted to communicate alternatelywith fluid pressure and exhaust during reciprocation of the piston sothat, with the spool at an end of its stroke, when the piston reaches anend of its stroke consistent with the spool in said end of strokeposition the closed control port communicates with fluid pressure andthe open control port and thereby itS respective spool-biasing chambercommunicates with exhaust and reversal of the spool causes the spool toopen the control port in communication with fluid pressure to itsrespective spool-biasing chamber to urge the spool to its other end ofstroke position.
 10. A motor as claimed in claim 9 wherein the twocontrol ports are so located and controlled by the spool that in noposition of the spool are they simultaneously closed to their respectivespool-biasing chambers.
 11. A motor as claimed in claim 9 wherein eachof the two control ports respectively communicates with one of two portsin the piston cylinder, each of the two ports in the piston cylinderbeing adapted to respectively communicate with one of the pair of pistonchambers so that with the spool at an end of stroke position the controlport which is closed by the spool communicates with the expandingpressure chamber and the control port which is open to its associatedspool-biasing chamber communicates with exhaust when the piston reachesits end of stroke position.
 12. A motor as claimed in claim 11 whereinthe two ports in the piston cylinder which communicate with the controlports are so located and controlled by the piston that in no position ofthe piston are they simultaneously closed to communication with theirrespectively associated piston chambers.
 13. A motor as claimed in claim11 in which the piston defines with its cylinder a piston transferchamber located axially between the pair of piston chambers and whereinsaid piston transfer chamber is in constant communication with exhaustand the two ports in the piston cylinder which communicate with thecontrol ports are so located and controlled by the piston that duringreciprocation of the piston each such port alternately communicates withits respectively associated piston chamber and the piston transferchamber.
 14. A motor as claimed in claim 13 in which the two ports inthe piston cylinder which communicate with the control ports are solocated and controlled by the piston that in no position of the pistonare they simultaneously open to communication with the piston transferchamber.
 15. A motor as claimed in claim 1 in which fluid pressure iscaused to be admitted to the other spool-biasing chamber by way of valvemeans responsive to axial movement of the piston.
 16. A motor as claimedin claim 15 wherein said valve means is adapted to be actuated to admitfluid pressure to the said other spool-biasing chamber immediatelybefore the port through which the contracting piston chamber exhausts isclosed by the piston.
 17. A motor as claimed in claim 15 in which thevalve means comprises cooperating ports and lands between the piston andits cylinder.
 18. A motor as claimed in claim 9 in which fluid pressureis caused to be admitted to the other spool-biasing chamber by way ofvalve means responsive to axial movement of the piston, and in which thespool cylinder has a pair of signalling ports each of which is adaptedto respectively communicate with one of the pair of spool-biasingchambers, the signalling ports being controlled by reciprocation of thespool to be alternately opened and closed to their respectivespool-biasing chambers and with the spool at an end of its stroke onesignalling port is open and the other signalling port is closed to itsrespective spool-biasing chamber, the open signalling port beingassociated with the spool-biasing chamber which, if subjected to fluidpressure, would urge the spool away from its said end of strokeposition; said signalling ports being adapted to communicate with fluidpressure and being controlled for such communication by way of saidvalve means; and wherein for each spool-biasing chamber, the signallingport and control port respectively associated therewith are so locatedand controlled by the spool that in no position of the spool are theysimultaneously closed to their respective spool-biasing chamber.
 19. Amotor as claimed in claim 18, wherein for eaCh spool-biasing chamber,the signalling port associated therewith is adapted to communicate byway of said valve means with fluid pressure at the control portassociated therewith.
 20. A motor as claimed in claim 1 in which fluidunder pressure is caused to be admitted to the other spool-biasingchamber by way of bleed passage means which is in constant communicationwith said other spool-biasing chamber and is adapted to communicate withfluid pressure during expansion of a piston chamber consistent with thespool at said end and to communicate with exhaust during contraction ofa piston chamber consistent with the spool at the other end of itsstroke.
 21. A motor as claimed in claim 9 in which fluid under pressureis caused to be admitted to the other spool-biasing chamber by way ofbleed passage means which is in constant communication with said otherspool-biasing chamber and is adapted to communicate with fluid pressureduring expansion of a piston chamber consistent with the spool at saidend and to communicate with exhaust during contraction of a pistonchamber consistent with the spool at the other end of its stroke, and inwhich each spool-biasing chamber and control port respectivelyassociated therewith are in constant communication by way of a bleedpassage.
 22. A motor as claimed in claim 8 in which fluid under pressureis caused to be admitted to the other spool-biasing chamber by way ofbleed passage means which is in constant communication with said otherspool-biasing chamber and is adapted to communicate with fluid pressureduring expansion of a piston chamber consistent with the spool at saidend and to communicate with exhaust during contraction of a pistonchamber consistent with the spool at the other end of its stroke, and inwhich each spool-biasing chamber and control port respectivelyassociated therewith are in constant communication by way of a bleedpassage.
 23. A motor as claimed in claim 1 wherein auxiliary valve meansis provided which is adapted to be actuated when the piston is locatedat or towards an end of stroke position and, with the spool at an end ofstroke position, actuation of said auxiliary valve means admits fluidpressure to the spool-working chamber which, when pressurized urges thespool to its other end of stroke position.
 24. A motor as claimed inclaim 23 in which means is provided by which said auxiliary valve meanscannot be actuated to admit fluid pressure to said spool-working chamberwhile said spool-working chamber communicates with exhaust.
 25. A motoras claimed in claim 5 wherein auxiliary valve means is provided which isadapted to be actuated when the piston is located at or towards an endof stroke position and, with the spool at an end of stroke position,actuation of said auxiliary valve means admits fluid pressure to thespool-working chamber which, when pressurized urges the spool to itsother end of stroke position, and in which, during axial movement of thepiston, the auxiliary valve means is adapted to be actuated to admitfluid pressure to said spool-working chamber only when the piston islocated at a position in which the port through which the contractingpiston chamber exhausts is closed by the piston from communication withsaid piston chamber.
 26. A motor as claimed in claim 25 in which theauxiliary valve means is adapted to admit fluid pressure into thecontracted piston chamber and therethrough into the spool-workingchamber in communication with the contracted piston chamber.
 27. A motoras claimed in claim 15, in which the valve means comprises cooperatingports and lands between a piston rod and a bore through which it isslidable.